1. Field of the Invention
The present invention relates to a flyback switching power supply device, and more particularly, to a switching power supply device that detects a voltage generated in a tertiary winding of a transformer and performs control on the basis of the detected voltage such that an output voltage from a secondary winding of the transformer is a predetermined value.
2. Description of the Related Art
In recent years, as a power supply device for a charger or a power supply device for an alternating current (AC) adapter of a notebook personal computer, a switching power supply device has been used which performs control such that a constant voltage is supplied to a load.
For example, U.S. Pat. No. 7,672,146 discloses this type of switching power supply device.
As illustrated in FIG. 5, the switching power supply device includes a transformer 2, a metal oxide semiconductor (MOS) transistor TR for switching, an output voltage generating unit 4, a feedback signal generating unit 6, and a switching control unit 8.
The transformer 2 includes a primary winding 2-1, a secondary winding 2-2, and a tertiary winding 2-3. A direct current (DC) voltage is applied to one end of the primary winding 2-1 of the transformer 2 and the other end of the primary winding 2-1 is connected to the drain of the MOS transistor TR. The source of the MOS transistor TR is connected to the ground through a resistor RS. The output voltage generating unit 4 includes a diode D1 and a capacitor C1, rectifies the voltage generated in the secondary winding 2-2 of the transformer 2, smoothes the rectified voltage, and outputs the smoothed voltage as an output voltage Vo.
The feedback signal generating unit 6 generates a feedback signal Vfb on the basis of a gate signal Vg input to the gate of the MOS transistor TR and a tertiary winding voltage Vt generated in the tertiary winding 2-3 of the transformer 2. The switching control unit 8 generates the gate signal Vg to be input to the gate of the MOS transistor TR on the basis of a voltage Vrs between both ends of the resistor RS and the feedback signal Vfb from the feedback signal generating unit 6.
Next, the operation of the switching power supply device having the above-mentioned structure will be described with reference to FIGS. 5 to 7C.
The switching control unit 8 outputs the gate signal Vg illustrated in FIG. 6A to the gate of the MOS transistor TR. In response to the gate signal Vg, the MOS transistor TR switches the DC voltage input to the primary winding 2-1 of the transformer 2. That is, the MOS transistor TR turns on or off a path from the DC voltage to the ground potential. Therefore, for a period from a time t1 to a time t2, the MOS transistor TR is turned on and the primary current Ip illustrated in FIG. 6B flows to the primary winding 2-1 of the transformer 2.
At the time t2, the MOS transistor TR is turned off and the secondary current Is illustrated in FIG. 6C starts to flow to the secondary winding 2-2 of the transformer 2. Then, as illustrated in FIG. 6D, the tertiary winding voltage Vt of the tertiary winding 2-3 of the transformer 2 is increased rapidly and is then monotonously decreased for a time t3.
For the period from the time t2 to the time t3, since the secondary current Is flows to the diode D1, the voltage Vs of the secondary winding 2-2, the output voltage Vo, and the forward voltage Vf of the diode D1 satisfy the following relationship: Vs=Vo+Vf.
The tertiary winding voltage Vt for the period is proportional to the voltage Vs of the secondary winding 2-2, and the forward voltage Vf of the diode D1 is changed by the current flowing to the diode D1. Therefore, when the output voltage Vo is estimated by the tertiary winding voltage Vt, an error occurs in the estimation of the output voltage Vo by the current flowing to the diode D1. Therefore, it is necessary to detect the tertiary winding voltage Vt when the current flowing to the diode D1 is very close to zero. The forward voltage Vf of the diode D1 when the current flowing to the diode D1 is zero is constant.
At the time t3, the secondary current Is is zero as illustrated in FIG. 6C, but the tertiary winding voltage Vt oscillates due to, for example, the parasitic capacitance of the MOS transistor TR or the excitation inductance of the transformer 2 as illustrated in FIG. 6D. In parallel to these operations, the feedback signal generating unit 6 generates the feedback signal Vfb as follows on the basis of the gate signal Vg of the MOS transistor TR and the tertiary winding voltage Vt of the transformer 2.
That is, at the time t2, when the gate signal Vg falls as illustrated in FIG. 6A, the feedback signal generating unit 6 starts the detection period T1 of the tertiary winding voltage Vt.
When the detection period T1 starts, the feedback signal generating unit 6 alternately samples and holds the tertiary winding voltage Vt using two sampling pulses PA and PB which are alternately generated at different times, as illustrated in FIGS. 7B and 7C.
Then, at the time t4, when it is detected that the tertiary winding voltage Vt crosses zero, the feedback signal generating unit 6 ends the detection period T1 for which the tertiary winding voltage Vt is sampled.
At the time t4 when the detection period T1 ends, there are a voltage which is sampled and held by the sampling pulse PA illustrated in FIG. 7B and a voltage which is sampled and held by the sampling pulse PB illustrated in FIG. 7C.
Then, the feedback signal generating unit 6 selects one of the two voltages which is sampled and held by the sampling pulse PA before the sampling pulse PB closest to the time t4 when the detection period ends, and outputs the selected voltage as the feedback signal Vfb.
However, in the switching power supply device disclosed in U.S. Pat. No. 7,672,146, when the time t3 has elapsed, a resonance operation starts on the primary side of the transformer 2. Therefore, as illustrated in FIG. 6D, the tertiary winding voltage Vt also starts to oscillate, and the oscillating frequency (resonance frequency) is changed depending on the parasitic capacitance of the MOS transistor TR or the excitation inductance of the transformer 2.
Since the parasitic capacitance of the MOS transistor TR depends on the level of the input voltage, the time when the detection period T1 ends depends on the level of the input voltage. Therefore, the resonance frequency of the tertiary winding voltage Vt varies depending on the input voltage, and the zero-cross time varies depending on the input voltage. As a result, the detection accuracy of the feedback signal Vfb output from the feedback signal generating unit 6 varies depending on the input voltage, and the output voltage Vo of the output voltage generating unit 4 is changed.
That is, when the tertiary winding voltage Vt starts to oscillate, the tertiary winding voltage Vt is changed regardless of the output voltage Vo. Therefore, in order to accurately estimate the value of the output voltage Vo, it is necessary to detect the tertiary winding voltage Vt at the time which is as close to the time t3 as possible, but the difference between the measurement point of time and the time t3 is changed by the input voltage. Therefore, an error occurs in the detection accuracy of the estimated value of the output voltage Vo, that is, the feedback signal Vfb.
As illustrated in FIGS. 7A to 7C, when the frequency of the sampling pulses PA and PB is high, a plurality of (two) sample and hold operations are performed for the period from the time t3 to the time t4, and it is difficult to perform detection in the vicinity of the time t3 at which the detection has been desired to be performed from the beginning. Therefore, it is difficult to increase the frequency of the sampling pulses PA and PB, and the value of the feedback signal Vfb output from the feedback signal generating unit 6 may not be proportional to the output voltage Vo, according to the relationship between the length of the detection period T1 and a sampling cycle TS.
Strictly, the value of the feedback signal Vfb being proportional to the output voltage Vo means that the feedback signal Vfb is a linear function of the output voltage Vo. Therefore, the term “proportion” includes the “linear function”. This holds for the following description.
The invention has been made in view of the above-mentioned problems and an object of the invention is to provide a switching power supply device capable of increasing a sampling frequency, reducing the dependence of an output voltage on an input voltage, and improving and stabilizing the detection accuracy of the output voltage.